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CHAPTER 2

CHAPTER 2. AMPLITUDE MODULATION (AM) 2.3 AM Single Side Band Communications. OBJECTIVES. To define and describe AM single sideband To compare single sideband transmission to conventional double sideband AM The explain the methods of generating SSB

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CHAPTER 2

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  1. CHAPTER 2 AMPLITUDE MODULATION (AM) 2.3 AM Single Side Band Communications

  2. OBJECTIVES • To define and describe AM single sideband • To compare single sideband transmission to conventional double sideband AM • The explain the methods of generating SSB • To define and describe AM vestigial sideband • To describe the advantages and disadvantages of SSB

  3. LECTURE OVERVIEW • Single sideband and double sideband • Power calculation in SSB • Method of generating SSB • Vestigial sidebands • Advantages and disadvantages of SSB

  4. fc-fm fc+fm fc Review: conventional AM(DSB-FC) Frequency spectrum: Bandwidth=2 x fmmax Total Power=Pcarrier +Pusb +Plsb

  5. Two major Disadvantages/Drawbacks of DSBFC • Large power consumption, where carrier power constitutes >2/3 transmitted power.{remember:carrier does not contain any information} • Large bandwidth utilized. Thus, DSBFC is both power and bandwidth inefficient

  6. fc-fm fc+fm fc Single Side Band Full Carrier (SSB-FC) Frequency spectrum: Bandwidth=fm(max) Total Power=Pcarrier +Pusb

  7. Cont’d… • A form of amplitude modulation in which the carrier is transmitted at full power but only one of the sidebands (either the upper or lower) is transmitted • Requires less bandwidth than DSBFC but also produces a demodulated signal with a lower amplitude

  8. fc-fm fc+fm fc Single Side band Suppress Carrier (SSB-SC) Frequency spectrum: Bandwidth=fm(max) Total Power=+Pusb

  9. Cont’d… • A form of amplitude modulation in which the carrier is totally suppressed and one of the sidebands removed. • Therefore, SSBSC requires half as much bandwidth as conventional DSB AM and considerably less transmitted power

  10. Comparison of time domain representation of three common AM transmission systems: Tomasi

  11. Example 1 • For an AM DSCFC wave with a peak unmodulated carrier voltage Vc = 10 Vp,frequency of 100kHz, a load resistor of RL = 10 , frequency of modulating signal of 10kHz and m = 1, determine the following • Powers of the carrier and the upper and lower sidebands. • ii) Total power of the modulated wave. • iii) Bandwidth of the transmitted wave. • iv) Draw the power and frequency spectrum.

  12. Example 1..cont’d • For the same given values, determine questions (ii)-(iv) for a AM DSB-SC, AM SSB-FC and AM SSB-SC systems. Determine also the percentage of power saved in each of the system design.

  13. Example 1..cont’d • Solution for DSBFC; i) ii) iii) Bandwidth=2xfmmax=2(10kHz)=20kHz

  14. 90kHz 110kHz Example 1..cont’d • Solution:For DSB-SC ii) iii)Bandwidth=2xfmmax=2(10kz)=20kHz iv)

  15. 100kHz fc-fm 110kHz Example 1..cont’d • Solution:For SSB-FC ii) iii)Bandwidth=fmmax=10kHz iv)

  16. Example 1..cont’d • Solution:For SSB-sC ii) iii)Bandwidth=fmmax=10kHz iv) 110kHz fc-fm fc

  17. Methods of Generating SSB i) Filtering method • A filter removes the undesired sideband producing SSB. • Balanced modulators is used to suppress the unwanted carrier and filters to suppress the unwanted sidebands • Quartz crystal filters are the most widely used sideband filters since they are very selective and inexpensive.

  18. Antenna Carrier oscillator DSB signal SSB signal Balanced modulator Sideband filter Linear amplifier Filter response curve Microphone Audio amplifier Lower sidebands Upper sidebands Block diagram of filtering method

  19. Cont’d… ii) Phasing method using two balance modulator • Another way to produce SSB uses a phase shift method to eliminate one sideband. • Two balanced modulators driven by carriers and modulating signals 90º out of phase produce DSB. • Adding the two DSB signals together results in one sideband being cancelled out.

  20. Am cos wmt A1(t) Balanced Modulator 1 Information signal Ac cos (wct + 90) Output Signal, aot Phase shifter + Carrier signal Phase shifter Balanced Modulator 2 A2(t) Am cos (wmt + 90) Block diagram of phasing method

  21. Mathematical analysis of phasing method

  22. VESTIGIAL SIDEBAND (VSB) • Also called asymmetric sideband system. • Compromise between DSB & SSB. • Easy to generate. • Bandwidth is only ~ 25% greater than SSB signals. • Derived by filtering DSB, one pass band is passed almost completely while just a trace or vestige of the other sideband is included.

  23. Cont’d • AM wave is applied to a vestigial sideband filter, producing a modulation scheme – VSB + C • Mainly used for television video transmission.

  24. VSB Carrier LSB MSB fc fc Cont’d…vsb • VSB Frequency Spectrum

  25. Advantages/Benefits of SSB • Power consumption • Bandwidth conservation • Selective fading • Noise reduction

  26. Disadvantages of SSB • Complex receivers • Tuning difficulties

  27. AT THE END OF THIS CHAPTER, YOU SHOULD BE ABLE, • To define and describe AM single sideband • To compare single sideband transmission to conventional double sideband AM • The explain the methods of generating SSB • To define and describe AM vestigial sideband • To describe the advantages and disadvantages of SSB

  28. END OF CHAPTER 2.3 AM SINGLE SIDEBAND COMMUNICATIONS

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